Ducted propeller systems for marine vessels

ABSTRACT

In a ducted propeller system for ship propulsion, comprising a propeller working in a fixed propulsion nozzle and rudder blades or shutters disposed at the nozzle entry on either side of the central vertical fore-and-aft plane of the nozzle, the rudder blades are toed out and the angle of the toeing is varied up the height of each blade to correspond to the different nozzle flow entry conditions that obtain at different regions of the nozzle entry due to the effect of propeller rotation.

United States Patent [151 3,640,07 1- Corlett Feb. 8, 1972 [54] DUCTED PROPELLER SYSTEMS FOR [56] References Cited UNITED STATES PATENTS [72] gxs 'gg Brew Cmlm Basmgsmke 2,139,594 12/1938 Kort ..60/22l ux 3,040,696 6/1962 Dahle.... [73] Assignee: Hydroconic Limted, Basingstoke, 3,083,529 4/ 1963 Hamilton ..60/22l Hampshire, England 22 d: Se L 29 1970 Primary Examiner-Clarence R. Gordon 1 1 e p Attorney-Dowel] & Dowell 21 Appl. No.: 76,508

[57] ABSTRACT Foreign Application Priority Data in a ducted propeller system for ship propulsion, comprising a Oct. 13 1969 Great Britain ..50 217/69 PmPeer waking a fixed PmPulsiO "Mlle and "udder blades or shutters disposed at the nozzle entry on either side of 52 us. (:1 ..60/22] the vertical Plame the "Mlle, the rudder Field of Search ..60/22l 222, 230, 231

blades are toed out and the angle of the toeing is varied up the height of each blade to correspond to the difi'erent nonle flow entry conditions that obtain at different regions of the nozzle entry due to the effect of propeller rotation.

3 Claims, 2 Drawing Figures 1 DUCTED PROPELLER SYSTEMS FOR MARINE VESSELS This invention relates to ducted propeller systems for the propulsion of marine vessels.

Ducted propeller systems may embody either a fixed nozzle or an angularly movable nozzle. If the latter, steering is effected by angular movement of the nozzle. If the former, rudders can be fitted at the after end of the nozzle for ahead steering and at the forward end of the nozzle for astem steering. In some systems two such rudder systems may work together; in other words, both forward and after rudders may operate for both ahead and astem steering. But in the more commonly employed systems the forward rudders are only used when the ship is going astern and the after rudders when the ship is going ahead.

If a propeller is designed for such a nozzle system so that each blade element operates at the optimum point on the lift drag ratio curve with respect to angle of incidence of inflow, then the propeller is wake adapted and is operating at its best possible efficiency. In such circumstances any downwash angle from the forward rudders, whether there be two, four or any other number, will tend to displace the operating point of a blade element from the peak of the lift drag ratio curve and cause deterioration in the efficiency of the propeller.

It is desirable to apply to the forward rudders an angle of incidence, with respect to the fore and aft direction parallel to the nozzle centerline, whether there be two, four or any other number of forward rudders. In the case of two forward rudders the leading edges of the rudders may be toed out by an angle of perhaps 4 or 5". In the case of multiple rudders or shutter vanes, as described in our patent specification No. 10296/69, the toeing angles of the inboard shutters should be less than those of the outboard shutters and of the order of the angles given in that patent specification.

According to the present invention, there is provided a ships ducted propeller system comprising a fixed propulsion nozzle and a propeller working therein, and nominally toed out rudder blades disposed at the nozzle entry on either side of the central vertical fore-and-aft plane of the nozzle, characterized in that the toeing angle of each rudder blade is different at different positions up its height to match the differing flow conditions at different regions of the nozzle entry.

The inflow conditions to a nozzle are different depending upon which quadrant of the entry disc is considered. For the purposes of this discussion a left-handed propeller is considered. With an accelerating nozzle the flow is contracted into the entrance of the nozzle and there is hence an inward component of flow transversely directed towards the axis of the propelling shaft. Recirculation of flow outside the nozzle from exit to entry enhances this effect and as a result at the top of the nozzle entry there is a downward component of flow as well as the axial one, at the bottom an upward component, at each of the two sides an inward component, and in intermediate positions a combination of these.

To determine the toeing angle of a vertical rudder blade we examine the horizontal components of flow. Considering the entry of the nozzle, and looking aft toward the left-handed propeller, at the top left-hand quadrant, i.e., between 9 oclock and 12 o'clock, there is an inward component due to the aforesaid recirculation around the nozzle section plus an inward component due to the rotation of the propeller plus the axial component due to the velocity of the ship and the propeller flow. Hence the resultant in this quadrant, of 9 to 12 oclock looking aft, is inward and the portion or portions of a flanking rudder or each shutter rudder in this quadrant should be toed outward at the leading edge.

In the diametrically opposite quadrant of the nozzle entry disc, namely from 3 to 6 oclock looking aft, the same considerations apply and the leading edge of the rudder or rudders should be toed out.

Considering the quadrant 6 to 9 oclock and its matching quadrant 12 to 3 o'clock looking aft, it is apparent that there is an inward component of flow due to nozzle recirculation and, with a left-handed propeller, an outward component due to rotation of the propeller. As a result these two radial components oppose each other and the toeing angle of the portions of the rudders within these quadrants should be materially different from and less than the toeing angle in the other two quadrants. In a particular instance, the two opposing flow components may cancel each other entirely, in which case these portions of the rudders should be disposed purely fore and aft; whereas in other instances the nozzle recirculation component may be greater than that of the propeller rotation, in which case a toeing out'angle should be applied; or if the nozzle recirculation is not as strong as the component due to propeller rotation the rudders should be toed in.

The exact toeing angle depends upon the calculation of these two effects and will vary from ship to ship and application to application.

To carry the invention into practice, each rudder or shutter vane may be given a continuous twist up its height, so that at any horizontal plane of the rudder the toeing angle is exactly matched to the balance of inward and outward flow components. Or, alternatively, in a simpler form, the rudder may be split so that at the top the rudder is toed one way, down to the horizontal diameter, and below that the rudder is toed to another angle. The junction between the two sections may be effected by a plate division.

The general steering characteristics otherwise are unchanged and the invention produces a considerable improvement in performance enabling the propeller designer to choose an optimum wake adapted screw for the particular nozzle and application without fear of its performance being significantly reduced by the effects of downwash angles from the rudders. Furthermore the drag of the rudders themselves is minimized as they are pointing in the true direction of flow. Thus, the performance of the screw is improved by this invention and also the parasitic drag of the rudders can be reduced to a minimum.

For a better understanding of the invention, reference should be had to the accompanying drawings, in which:

FIG. 1 shows a ships ducted propeller system in diagrammatic longitudinal section, and

FIG. 2 is a diagram of the nozzle entry looking aft, to illustrate the entry conditions.

In the drawings, a propeller 11 rotates, in the direction of the arrows F, in a fixed nozzle 12 provided at its entry with rudder blades 13. The nozzle entry flow, ignoring propeller rotation, is indicated by the lines 14, and superimposed on this is the horizontal component of flow, indicated by the lines 15, due to propeller rotation this being in some places toward the central fore-and-aft vertical plane of the nozzle and in some places away from it.

Considering the entry of the nozzle divided into four quadrants as previously discussed, FIG. 2 shows in each quadrant of the nozzle entry a respective typical vector diagram of the horizontal flow components. In the top left-hand quadrant, i.e., between 9 and 12 oclock, there is the inward component C due to the recirculation around the nozzle plus the inward component R due to the rotation of the propeller, together with the axial component A due to the velocity of the ship and the propeller flow. The resultant B in this quadrant is inward and therefore the portion of the flanking rudder 13 in this quadrant is toed outward at its leading edge in accordance with the angle made by the vector B to the fore-and-aft center line of the nozzle, as indicated at 13a.

In the diametrically opposite quadrant of the nozzle entry, namely from 3 to 6 oclock, the same conditions obtain and the respective portion of the other rudder is toed out at the same angle.

However, in the quadrant 6 to 9 oclock and its matching quadrant 12 to 3 O'clock, the inward component of flow C due to nozzle recirculation is opposed by the component R due to rotation of the propeller which is now outward. Consequently, the angle of the resultant vector B to the fore-and-aft center line is much smaller and the portions of the rudders l3 lying in these quadrants are toed out to this correspondingly smaller angle, as indicated at 13!).

2. A system according to claim 1, wherein each rudder blade is twisted so that the toeing angle changes progressively up its height.

3. A system according to claim 1, wherein each rudder blade is divided at the horizontal diametral plane of the nozzle entry, the portion of the blade above this plane having a different toeing angle to the portion below. I 

1. A ship''s ducted propeller system comprising a fixed propulsion nozzle and a propeller working therein, and nominally toed out rudder blades disposed at the nozzle entry on either side of the central vertical fore-and-aft plane of the nozzle, characterized in that the toeing angle of each rudder blade is different at different positions up its height to match the differing flow conditions at different regions of the nozzle entry.
 2. A system according to claim 1, wherein each rudder blade is twisted so that the toeing angle changes progressively up its height.
 3. A system according to claim 1, wherein each rudder blade is divided at the horizontal diametral plane of the nozzle entry, the portion of the blade above this plane having a different toeing angle to the portion below. 